1. Field of the Invention
This invention relates to a method for forming halftone data for flat bed type color scanners or the like, and more particularly to a method for forming the halftone data which takes into consideration the difference in halftone screen percentage of respective dots in order to avoid a tone jump when image signals are obtained by scanning an original comprising color images in continuous tone, and are superposed with halftone screen signals which are electrically generated so as to form four-color separated halftone gradation images in C (cyan), M (magenta), Y (yellow) and K (black).
2. Description of the Prior Art
In the fields of printing and plate-making, the image scanning/reading/recording/systems which electrically process image information of an original for forming original film plates for printing have been widely used in recent years in order to rationalize the process and improve the image quality.
Such an image scanning/reading/recording system comprises basically an input section, a controller and an output section. More particularly, the system includes an input section where image signals are picked up by the illumination, color separation and photometric systems to convert image information photoelectrically, and the controller which processes the information in arithmetic operation such as gradation correction, color correction, contour emphasis, conversion from R (red), G (green), B (blue).fwdarw.C, M, Y, K and the like depending on the plate-making conditions. The processed image information is converted into optical signals such as laser beams by the output section to be printed in the form of images on a recording medium comprising a photosensitive material. The medium is developed by a predetermined developing device and used for printing and so on as the original film plates.
When the original to be printed is a photograph or a painting of continuous tone images, the original should be divided into dots to express the density of images. The continuous tone images are therefore transformed into halftone gradation images which are a group of dots in sizes which are different depending on the density thereof. As a method for forming such dots, there has been proposed a method which irradiates optical signals depending on the continuous tone images on a recording medium via a contact screen placed on a film. The contact screen comprises arrays of dots with blurred circumferences. There is usually employed in the image scanning/reading/recording system a method which electrically forms a halftone screen which is equivalent to the contact screen.
A good example of the prior art methods of forming a halftone screen, disclosed in Japanese Patent Publication (kokoku) No. 49361/1977, is briefly described below.
In FIG. 1, the reference numeral 100 denotes a basic periodic section of the halftone screen which is electrically formed. The halftone screen formed by repetition of the same pattern, and the minimum unit thereof is the basic periodic section 100. The basic periodic section 100 comprises eight scanning lines S.sub.1, through S.sub.8 arranged in parallel to each other in the direction of Y axis. The scanning lines S.sub.1 through S.sub.8 form respectively parts of the basic periodic 100 with unique voltage signals which change along the recording direction X. The respective voltages of the scanning lines S.sub.1, S.sub.2, S.sub.4 and S.sub.5 are set at a high level when they are passing through point A and point D in a dot section 101 while the voltage of the scanning line S.sub.3 is set at a low level when it passes through the point E. The voltages of the respective scanning lines S.sub.1 through S.sub.5 are set to gradually decrease from the points A through D toward the point E. The voltage signals at the scanning lines S.sub.1 through S.sub.8 may be formed into the halftone screen signals by, for example, superposing plural alternating voltage signals of a triangle shape of different periods and gradually shifting their phases for each scanning line.
When multicolor images and so on are formed into dots for reproduction, it is necessary to generate plural halftone screens and to superpose the halftone gradation images which are formed by the plural halftone screens. The halftone screens are formed respectively in the form rotated from the recording direction X at a predetermined angle .theta. in order to prevent generation of Moire patterns at the time of superposing.
A basic periodic section 100 which is formed as above is generated periodically at a frequency sufficient to cover the scanning area of the original to form the halftone screen. The halftone screen signals forming such a halftone screen are superposed with the image signals which are optically read from the original at the input section of the image scanning/reading/recording system in order to form the halftone gradation images on the original film plate.
A halftone plate of an area modulation type for color printing is characterized by the number of screen lines (e.g. the number of lines/inch: LPI), the screen angle (m/n) and the dot patterns. The screen angle is a rational number defined by m/n of FIG. 1, and is required for each of four colors of the four colors of C, M, Y and K. The screen angle and the dot patterns are formed with the method disclosed in Japanese Patent Publication No. 49361/1977 mentioned above. An arbitrary number of the screen lines may easily be obtained in the drum type color scanners simply by changing the imaging magnification with optical zooming, but in the flat bed (plane) type color scanners, the optical zooming is quite difficult. In the color scanners of the flat bed type, it is necessary to optically conduct main scanning of a light spot due to its high speed processing, and it is almost impossible to further add an additional mechanism for zooming to the system. Moreover, the control system has to be made bigger and more complex in order to change the size of the light spot or pitches.
It is further necessary to prevent generation of a tone jump or coupling of adjacent blackened portions of the dots.